U.S. patent application number 11/801284 was filed with the patent office on 2007-11-22 for control system, process and apparatus for hydrogen production from reforming.
Invention is credited to Kishore J. Doshi, John R. Harness.
Application Number | 20070269690 11/801284 |
Document ID | / |
Family ID | 38712339 |
Filed Date | 2007-11-22 |
United States Patent
Application |
20070269690 |
Kind Code |
A1 |
Doshi; Kishore J. ; et
al. |
November 22, 2007 |
Control system, process and apparatus for hydrogen production from
reforming
Abstract
A hydrogen generator contains a membrane separator and a
pressure swing sorption system to produce two hydrogen product
streams of differing purity. One of those streams is used as a feed
to a fuel cell to generate electricity and the other is used as the
primary hydrogen product.
Inventors: |
Doshi; Kishore J.;
(Fernandina Beach, FL) ; Harness; John R.; (Elgin,
IL) |
Correspondence
Address: |
Pauley Petersen & Erickson
Suite 365, 2800 West Higgins Road
Hoffman Estates
IL
60169
US
|
Family ID: |
38712339 |
Appl. No.: |
11/801284 |
Filed: |
May 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60802357 |
May 22, 2006 |
|
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|
Current U.S.
Class: |
48/197R ;
422/187; 422/211; 423/651; 429/411; 429/412; 429/420; 429/423 |
Current CPC
Class: |
C01B 3/503 20130101;
C01B 2203/043 20130101; C01B 2203/0485 20130101; C01B 2203/1247
20130101; C01B 2203/0405 20130101; C01B 3/382 20130101; C01B
2203/066 20130101; C01B 3/386 20130101; C01B 3/505 20130101; C01B
2203/0244 20130101; Y02E 60/50 20130101; C01B 3/56 20130101; C01B
2203/048 20130101; C01B 2203/0233 20130101; C01B 2203/0261
20130101; C01B 3/38 20130101; C01B 2203/1685 20130101; C01B
2203/0283 20130101; C01B 2203/1628 20130101; C01B 2203/0288
20130101; C01B 2203/047 20130101; C01B 2203/1241 20130101; C01B
2203/1623 20130101; C01B 2203/0475 20130101; H01M 8/0618 20130101;
C01B 2203/0495 20130101; C01B 2203/127 20130101; C01B 2203/145
20130101 |
Class at
Publication: |
429/19 ; 422/187;
422/211; 423/651 |
International
Class: |
H01M 8/06 20060101
H01M008/06; B01J 8/00 20060101 B01J008/00; C01B 3/38 20060101
C01B003/38; C01B 3/50 20060101 C01B003/50 |
Claims
1. A process for generating hydrogen and electrical power
comprising: a. reforming under catalytic reforming conditions
including elevated temperature and the presence of steam and fuel
to produce a reformate containing hydrogen, steam, carbon monoxide
and carbon dioxide; b. contacting at a pressure substantially no
greater than that of the reformate of step a at least a portion of
the reformate with a membrane selective for the permeation of
hydrogen as compared to steam, carbon monoxide and carbon dioxide
under permeation conditions including a temperature sufficient to
prevent condensation of steam to permeate up to about 50 mole
percent of the hydrogen contained in the portion of the reformate
contacting the membrane to provide a first hydrogen product and to
provide a retentate fraction; c. subjecting the retentate fraction
and any portion of the reformate not subjected to step b to
pressure swing sorption to provide a second hydrogen product
containing at least about 90 volume percent hydrogen and a purge
fraction; and d. reacting at least a portion of one of the first
hydrogen product and the second hydrogen product in a fuel cell to
produce electricity and providing the other of the first hydrogen
product and the second hydrogen product as a primary hydrogen
product.
2. The process of claim 1 wherein substantially all of the
reformate is subjected to step b.
3. The process of claim 1 wherein the reformate is split into a
permeator feed fraction and a retained reformate fraction, said
permeator feed fraction comprising up to about 50 volume percent of
the reformate, and is the portion of the reformate contacting the
membrane of step b.
4. The process of claim 3 wherein the first hydrogen product is
reacted in a fuel cell and the first hydrogen product contains less
than about 20 ppmv carbon monoxide.
5. The process of claim 3 wherein the reformate is subjected to
water gas shift conditions to provide a shift effluent containing
an increased concentration of hydrogen and a reduced concentration
of carbon monoxide.
6. The process of claim 3 wherein step b is prior to subjecting the
reformate to water gas shift conditions.
7. The process of claim 3 wherein step b is subsequent to
subjecting the reformate to water gas shift conditions.
8. A process for controlling the volume of hydrogen production from
a hydrogen generator comprising: a. reforming under catalytic
reforming conditions including elevated temperature and the
presence of steam and fuel to produce a reformate containing
hydrogen, steam, carbon monoxide and carbon dioxide; b. contacting
at a pressure substantially no greater than that of the reformate
of step a at least a portion of the reformate with a membrane
selective for the permeation of hydrogen as compared to steam,
carbon monoxide and carbon dioxide under permeation conditions
including a temperature sufficient to prevent condensation of steam
to permeate up to about 50 mole percent of the hydrogen contained
in the portion of the reformate contacting the membrane to provide
a first hydrogen product and to provide a retentate fraction; c.
subjecting the retentate fraction and any portion of the reformate
not subjected to step b to pressure swing sorption to provide a
second hydrogen product containing at least about 90 volume percent
hydrogen and a purge fraction; d. reacting at least a portion of
one of the first hydrogen product and the second hydrogen product
in a fuel cell to produce electricity and providing the other of
the first hydrogen product and the second hydrogen product as a
primary hydrogen product; e. determining the demand for the primary
hydrogen product; and f. providing a driving force for the
permeation of hydrogen in step b sufficient to permeate an amount
of hydrogen such that the primary hydrogen product is in an amount
substantially equivalent to the demand.
9. The process of claim 8 wherein the absolute pressure drop across
the membrane is used to provide the sought driving force for step
f.
10. The process of claim 8 wherein the flow rate of the reformate
per unit area of membrane is used to provide the sought driving
force for step f.
11. The process of claim 8 wherein the first hydrogen product is
reacted in a fuel cell and first hydrogen product contains less
than about 20 ppmv carbon monoxide.
12. A hydrogen generator comprising: a. a reformer containing
reforming catalyst and adapted to provide under catalytic reforming
conditions including elevated temperature and the presence of steam
and fuel, a reformate containing hydrogen, steam, carbon monoxide
and carbon dioxide; b. a membrane separator having a retentate side
and a permeate side wherein the retentate side is in fluid
communication with the reformate splitter to receive at least a
portion of the reformate for contact with a membrane selective for
the permeation of hydrogen as compared to steam, carbon monoxide
and carbon dioxide to provide on the permeate side a first hydrogen
product and on the retentate side a retentate; c. a pressure swing
sorption system in fluid communication with the reformer and
adapted to receive reformate and in fluid communication with the
retentate side of the membrane separator, said pressure swing
sorption system being adapted to provide a second hydrogen product
containing at least about 90 volume percent hydrogen and a purge
fraction; and d. a fuel cell in fluid communication with one of the
permeate side of the membrane separator and the pressure swing
sorption system, adapted to generate electricity by reacting
hydrogen.
13. The hydrogen generator of claim 12 further comprising a
reformate splitter in fluid flow communication with the reformer
for receiving reformate adapted to provide a permeator feed
fraction in fluid communication with the retentate side of the
membrane separator and a retained reformate fraction in fluid
communication with the pressure swing sorption system.
14. The hydrogen generator of claim 12 further comprising a water
gas shift reactor in fluid flow communication between the reformer
and the pressure swing sorption system.
15. The hydrogen generator of claim 14 in which the reformate
splitter is positioned between the reformer and the water gas shift
reactor.
16. The hydrogen generator of claim 14 in which the reformate
splitter is positioned between the water gas shift reactor and the
pressure swing sorption system.
17. The hydrogen generator of claim 16 further comprising a low
temperature water gas shift reactor positioned between the membrane
separator and the pressure swing sorption system and in fluid
communication with the retentate side of the membrane separator.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/802,357, filed 22 May 2006.
FIELD OF THE INVENTION
[0002] This invention pertains to control systems for operating
reformers for the generation of hydrogen, to processes for
reforming to produce a primary hydrogen product and electricity,
and to apparatus therefore, and in particular to such processes
purifying hydrogen generated by reforming through integrated
membrane and pressure swing sorption unit operations.
BACKGROUND TO THE INVENTION
[0003] Reforming of fuels is a well known process for generating
hydrogen. Hydrogen is used for various purposes including as a
chemical reactant, an annealing atmosphere and fuel to a fuel cell
for generating electricity. Reforming processes include steam
reforming, partial oxidation reforming and autothermal reforming,
all of which use one or more catalysts.
[0004] As the fuels comprise components containing hydrogen and
carbon such as hydrocarbons and oxygenated hydrocarbons, e.g.,
alcohols, ethers and the like, each of these types of reforming
processes co-produce carbon oxides (carbon monoxide and carbon
dioxide), the reformate is subjected to one or more unit operations
to remove carbon oxides. Most often, the reformate is subjected to
water gas shift conditions to generate additional hydrogen and
carbon dioxide from carbon monoxide and steam contained in the
reformate. The reformate may contain other impurities. For
instance, with partial oxidation and autothermal reforming using
air or oxygen enriched air, the reformate will contain nitrogen and
argon.
[0005] The intended use for the hydrogen product defines acceptable
hydrogen product compositions. For instance, where the hydrogen
product is to be used as the fuel for a PEM-type fuel cell, the
concentration of carbon monoxide, a poison to the fuel cell, is
preferably less than about 10, more preferably less than about 5,
ppmv (parts per million by volume). Nitrogen and carbon dioxide, in
such applications, primarily act as diluents. The extent to which
these components are removed is thus one of economics. Where
hydrogen is used as an annealing atmosphere, the presence of carbon
monoxide and carbon dioxide is generally not desired in that
carbonization may occur under annealing conditions and adversely
affect the work piece. In chemical processes, the specifications of
the hydrogen will depend upon the nature of the process as well as
any catalysts used therein.
[0006] Proposed methods for purifying reformates to provide a
hydrogen product have included water gas shift, selective oxidation
to oxidize carbon monoxide to carbon dioxide, thermal swing
sorption, pressure swing sorption and selective permeation through
a membrane. Selective permeation has not met with much commercial
acceptance due to the combination of partial pressure differential
driving force and membrane surface areas required for recovery of a
desired percentage of hydrogen in the reformate. To reduce membrane
surface area for a given percentage of hydrogen recovery, an
increase in the partial pressure differential is required. Any
compression of hydrogen-containing streams is energy intensive.
Work, nevertheless, is continuing to develop suitable membranes.
See, for instance, "Membranes for Gas Separation", Chemical and
Engineering News, Oct. 3, 2005, pp. 49 to 57, at pages 53 and
55.
[0007] Steam reforming of hydrocarbon-containing feedstock is a
conventional source of hydrogen. Steam reforming of hydrocarbons is
practiced in large-scale processes, often integrated with refinery
or chemical operations. Thus, integrated reforming and chemical
operations, due to their large scale and available skilled labor
force, can rely upon sophisticated unit operations to economically
produce hydrogen.
[0008] Hydrogen is difficult to store and distribute and has a low
volumetric energy density compared to hydrocarbon-containing fuels.
Thus, it is desirable to be able to generate hydrogen for use or
distribution at a point proximate to the consumer such that a
hydrocarbon-containing feedstock to the hydrogen generator is the
material shipped and stored. However, the demand for hydrogen at
such use or distribution points may be relatively small. Much
greater challenges exist in producing hydrogen in smaller scale
units than for the large industrial-scale hydrogen generators.
Moreover, it is likely consumers who draw from a smaller scale
hydrogen generator will not have constant demand. Hence, the
hydrogen generator must be capable of changing hydrogen production
rate.
[0009] Changing hydrogen production rates, however, is complex
given the number of unit operations involved in reforming and
purifying the reformate as well as the need to meet hydrogen
product specifications including during the transition between
hydrogen production rates. Where reformer outputs are intended to
change, the use of partial oxidation reforming and autothermal
reforming have been preferred since those types of reformers more
readily lend themselves to changes than do steam reformers which
must have the amount of externally provided heat change.
[0010] Heretofore it has been proposed to operate small reformers
to generate electricity for, e.g., household use. When the
electrical demand for electricity is down, the excess hydrogen
could be stored, or more preferably used as a source of heat.
Another proposal has been to divert a portion of the hydrogen
product for generating electricity while the remaining portion of
the purified reformate is used for alternative purposes such as for
chemical operations. See, for instance, International Publication
Number WO 2005/009892 A2, corresponding to International
Publication Number PCT/US2004/23707. The problem, however, is that
the entire reformate must be able to meet the stringent purity
specifications required for most types of fuel cells.
SUMMARY OF THE INVENTION
[0011] In accordance with this invention, a hydrogen generator is
provided with two hydrogen purification systems for treating
reformer effluent, one, a membrane separator, for providing
hydrogen of a first purity and the other, a pressure swing sorption
system, for providing hydrogen of a second purity. Only a fraction
of the hydrogen in the feed to the membrane separator permeates the
membrane, and those gases not permeating are provided to the
pressure swing sorption system for additional recovery. Thus, by
removing only a portion of the hydrogen as permeate in the membrane
separator, an attractive driving force for permeation of hydrogen
can be maintained such that lower membrane surface area is required
for a given flux. Accordingly, the pressure of the reformate need
not be compressed to provide an adequate partial pressure driving
force across the membrane. Hence, the hydrogen generator and
processes of this invention take advantage of a membrane separation
to provide a hydrogen stream of a first purity while still
achieving adequate hydrogen recovery through pressure swing
sorption purification.
[0012] The invention is particularly useful where partial oxidation
or autothermal reforming using air as the oxygen source provides
the reformate hydrogen. Not only are these types of reforming more
suitable than steam reforming where lower volumes of hydrogen
product are sought, but also the hydrogen purification systems can
handle the presence of nitrogen to provide suitable purity hydrogen
products for fuel cell and for primary use applications.
[0013] The hydrogen from the membrane separation, i.e., the
hydrogen of the first purity, will have a purity that in part
relates to the selectivity of the membrane. The hydrogen from the
membrane separation may be of higher or lower purity than that from
the pressure swing sorption. The advantages of having two
separately purified hydrogen products are multifold. One of the
products can be of appropriate purity for use as a fuel to a fuel
cell to generate electricity while the other may have a greater or
lesser purity for the primary use of the hydrogen product. Hydrogen
for use in fuel cells such as PEM fuel cells, typically must have a
very low concentration of carbon monoxide, a poison for the fuel
cells. But fuel cells can tolerate the presence of nitrogen and
carbon dioxide. The primary use of the hydrogen may have much
different purity requirements. Hydrogen for electronic use, for
instance, has to be highly pure and even nitrogen is not tolerated.
Whereas for float glass, annealing and some chemical processes, the
presence of hydrogen and some carbon oxides may be acceptable. The
primary use of the hydrogen may be for storage for subsequent
refueling of vehicles using hydrogen as fuel.
[0014] In a preferred aspect, the hydrogen generator of this
invention can readily be operated to accommodate changes in demand
for hydrogen for the primary use while still enabling the reformer
to operate at a given production level, or alternatively or in
addition, to facilitate a turn up or turn down in hydrogen
production by the reformer. In this preferred aspect, the amount of
hydrogen that is withdrawn as permeate in the membrane separator is
changed by changing at least one of the pressure drop and flow rate
of effluent to the membrane separator thereby affecting the split
between the hydrogen product of the first purity and the hydrogen
product of the second purity. While the rate of electricity
generation by the fuel cell will fluctuate, those changes are
capable of being easily accommodated. For instance, if the fuel
cell is associated with an electrical power grid, the excess
electrical power can be placed in the grid or it can be used for
other purposes. Where the electricity generated by the fuel cell is
used to power the hydrogen generator, external power, such as from
a power grid or battery, can be used to supplement internal needs
for electrical power where the demand for the primary hydrogen
product results in a shortage of hydrogen product for the fuel
cell.
[0015] The broad aspects of the process of this invention for
generating hydrogen and electrical power comprise: [0016] a.
reforming under catalytic reforming conditions including elevated
temperature and the presence of steam a fuel to produce a reformate
containing hydrogen, steam, carbon monoxide and carbon dioxide;
[0017] b. contacting at a pressure substantially no greater than
that of the reformate of step a at least a portion of the reformate
with a membrane selective for the permeation of hydrogen as
compared to steam, carbon monoxide and carbon dioxide under
permeation conditions including a temperature sufficient to prevent
condensation of steam to permeate up to about 50 mole percent of
the hydrogen contained in the portion of the reformate contacting
the membrane to provide a first hydrogen product and to provide a
retentate fraction; [0018] c. subjecting the retentate fraction and
any portion of the reformate not subjected to step b to pressure
swing sorption to provide a second hydrogen product containing at
least about 90 volume percent hydrogen and a purge fraction; and
[0019] d. reacting at least a portion of one of the first hydrogen
product and the second hydrogen product in a fuel cell to produce
electricity and providing the other of the first hydrogen product
and the second hydrogen product as a primary hydrogen product.
[0020] In a preferred aspect of the invention, the reformate is
split into a permeator feed fraction and a retained reformate
fraction, said permeator feed fraction comprising up to about 50
volume percent of the reformate, and is the portion of the
reformate contacting the membrane of step b. In further preferred
aspects of the processes of the invention, the first hydrogen
product is reacted in a fuel cell and first hydrogen product
contains less than about 20 ppmv carbon monoxide.
[0021] The broad aspects of this invention relating to a process
for controlling the volume of hydrogen production comprise: [0022]
a. reforming under catalytic reforming conditions including
elevated temperature and the presence of steam a fuel to produce a
reformate containing hydrogen, steam, carbon monoxide and carbon
dioxide; [0023] b. contacting at a pressure substantially no
greater than that of the reformate of step a at least a portion of
the reformate with a membrane selective for the permeation of
hydrogen as compared to steam, carbon monoxide and carbon dioxide
under permeation conditions including a temperature sufficient to
prevent condensation of steam to permeate up to about 50 mole
percent of the hydrogen contained in the portion of the reformate
contacting the membrane to provide a first hydrogen product and to
provide a retentate fraction; [0024] c. subjecting the retentate
fraction and any portion of the reformate not subjected to step b
to pressure swing sorption to provide a second hydrogen product
containing at least about 90 volume percent hydrogen and a purge
fraction; [0025] d. reacting at least a portion of one of the first
hydrogen product and the second hydrogen product in a fuel cell to
produce electricity and providing the other of the first hydrogen
product and the second hydrogen product as a primary hydrogen
product; [0026] e. determining the demand for the primary hydrogen
product; and [0027] f. providing a driving force for the permeation
of hydrogen in step b sufficient to permeate an amount of hydrogen
such that the primary hydrogen product is in an amount
substantially equivalent to the demand.
[0028] The driving force may be adjusted by any suitable means
including adjusting the absolute pressure drop across the membrane,
and by changing the flow rate of the reformate per unit of membrane
surface area. By any of these methods, a change in flow rate of
primary hydrogen product can be achieved.
[0029] The hydrogen generators of this invention, in the broad
aspects, comprise: [0030] a. a reformer containing reforming
catalyst and adapted to provide under catalytic reforming
conditions including elevated temperature and the presence of steam
a fuel a reformate containing hydrogen, steam, carbon monoxide and
carbon dioxide; [0031] b. a membrane separator having a retentate
side and a permeate side wherein the retentate side is in fluid
communication with the reformate splitter to receive at least a
portion of the reformate for contact with a membrane selective for
the permeation of hydrogen as compared to steam, carbon monoxide
and carbon dioxide to provide on the permeate side a first hydrogen
product and on the retentate side a retentate; [0032] c. a pressure
swing sorption system in fluid communication with the reformer and
adapted to receive reformate and in fluid communication with the
retentate side of the membrane separator, said pressure swing
sorption system being adapted to provide a second hydrogen product
containing at least about 90 volume percent hydrogen and a purge
fraction; and [0033] d. a fuel cell in fluid communication with one
of the permeate side of the membrane separator and the pressure
swing sorption system, adapted to generate electricity by reacting
hydrogen.
[0034] The apparatus preferably further comprises a reformate
splitter in fluid flow communication with the reformer for
receiving reformate adapted to provide a permeator feed fraction in
fluid communication with the retentate side of the membrane
separator and a retained reformate fraction in fluid communication
with the pressure swing sorption system. In yet another preferred
aspect of the invention, the apparatus comprises at least one water
gas shift reactor in fluid communication with and located between
the reformer and the pressure swing sorption assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic representation of a hydrogen generator
of this invention in which the reformate splitter is positioned
between the reformer and a water gas shift reactor and the
retentate from the permeator is returned downstream of the splitter
but before the water gas shift reactor.
[0036] FIG. 2 is a schematic representation of a hydrogen generator
of this invention in which the reformate splitter is positioned
downstream of a water gas shift reactor and the retentate from the
permeator is returned downstream of the splitter.
[0037] FIG. 3 is a schematic representation of a hydrogen generator
similar to that depicted in FIG. 2 but with the permeate being
subjected to a low temperature water gas shift prior to being
recombined with the retained fraction from the splitter.
[0038] FIG. 4 is a schematic representation of a hydrogen generator
similar to that depicted in FIG. 2 in which two water gas shift
reactors are employed.
DETAILED DISCUSSION
[0039] With reference to FIG. 1, reforming feed is provided through
one or more lines 102 to reformer 104. The feeds to a reformer will
depend upon the type of reforming to be effected, which may be
partial oxidation, autothermal reforming (ATR) or steam reforming.
For partial oxidation and ATR reforming, the feed will include an
oxygen source such as air, oxygen-enriched air or substantially
pure oxygen. Typically where an oxygen source is required, it is
air or oxygen-enriched air, e.g., up to about 90, more frequently,
up to about 50, volume percent free oxygen. The amount of oxygen
provided for partial oxidation and ATR reforming will again be
dependent on the type of reforming process. For ATR, the amount of
oxygen is sufficient to generate sufficient heat through combustion
to maintain desired reforming temperatures. For partial oxidation
reforming, the amount of oxygen will be based on stoichiometry.
Preferably, the reforming is partial oxidation reforming, and more
preferably ATR, using air as the oxygen source.
[0040] The fuel for reforming may be any suitable
hydrocarbon-containing component and are typically gaseous under
the conditions of reforming. Lower hydrocarbon gases such as
methane, ethane, propane, butane and the like may be used. Because
of availability, natural gas and liquid petroleum gas (LPG) are
most often used as feeds. Oxygenated hydrocarbon-containing feeds
such as methanol and ethanol are included as hydrocarbon-containing
feeds for all purposes herein.
[0041] Natural gas and liquid petroleum gas typically contain
odorants such that leaks can be detected. Odorants conventionally
used are one or more organosulfur compounds such as organosulfides,
e.g., dimethyl sulfide, diethyl sulfide, and methyl ethyl sulfide;
mercaptans, e.g., methyl mercaptan, ethyl mercaptan, and t-butyl
mercaptan; thiophenes of which tetrahydrothiophene is the most
common; and the like. The amount used can vary widely. For natural
gas, the organosulfur component is often in the range of about 1 to
20 parts per million by volume (ppmv); and for LPG a greater amount
of sulfur compounds are typically used, e.g., from about 10 to 200
ppmv. It is not unusual for commercially obtained hydrocarbon feeds
to contain also other sulfur compounds which may be natural
impurities such as hydrogen sulfide and carbonyl sulfide. Carbonyl
sulfide concentrations in natural gas and LPG of 0.1 to 5 ppmv are
not unusual. Regardless of the form of the sulfur, it can be
deleterious to catalysts used in hydrogen generators and to fuel
cells. Accordingly, the feed should be desulfurized. Any convenient
desulfurization technique may be used including sorption and
hydrodesulfurization. Desulfurization may, if desired, be effected
on the reformer effluent since reforming catalysts do exist that
can tolerate some amount of sulfur. One advantage of conducting the
desulfurization after reforming is that the reforming reactions
convert sulfur components into hydrogen sulfide.
[0042] The feeds can contain other impurities such as carbon
dioxide, nitrogen and water. In the processes of this invention, it
is preferred that the concentration of carbon dioxide in the feed
be less than about 5, preferably less than about 2, volume
percent.
[0043] The pressure in the reforming conditions of the processes of
this invention is at least about 400 kPa, say from about 500 kPa to
1500 or 2500 kPa, preferably from about 500 kPa to about 1200 kPa,
absolute. Thus the reforming conditions comprise a pressure
suitable for the operation of the pressure swing sorption system
and the membrane separator without an intervening compression. The
reforming may be via steam reforming alone or may be effected by
partial oxidation or by a combination of partial oxidation of the
fuel being passed to the reformer and steam reforming (ATR). Steam
reforming is a catalytic reaction producing hydrogen and carbon
oxides (carbon dioxide and carbon monoxide) conducted under steam
reforming conditions. Steam reforming conditions usually comprise
temperatures in excess of 600.degree. C., e.g., 600.degree. C. to
1000.degree. C.
[0044] Partial oxidation reforming conditions typically comprise a
temperature of from about 600.degree. C. to about 1000.degree. C.,
preferably about 600.degree. C. to 800.degree. C. The partial
oxidation reforming is catalytic. The overall partial oxidation and
steam reforming reactions for methane are expressed by the
formulae:
CH.sub.4+0.5O.sub.2.fwdarw.CO+2H.sub.2
CH.sub.4+H.sub.2OCO+3H.sub.2
[0045] The reformer may comprise two discrete sections, e.g., a
first contact layer of oxidation catalyst followed by a second
layer of steam reforming catalyst, or may be bifunctional, i.e.,
oxidation catalyst and steam reforming catalyst are intermixed in a
single catalyst bed or are placed on a common support. The partial
oxidation reformate comprises hydrogen, nitrogen (if air is used as
the source of oxygen), carbon oxides (carbon monoxide and carbon
dioxide), steam and some unconverted hydrocarbons.
[0046] The reformate contains hydrogen, carbon dioxide and carbon
monoxide as well as water. On a dry basis, the components of the
effluent from the reformer fall within the ranges set forth
below:
TABLE-US-00001 REFORMER EFFLUENT COMPONENTS, MOLE PERCENT DRY BASIS
Component Steam Reforming Autothermal Reforming Hydrogen 50 to 80,
35 to 60, frequently 40 to 45 frequently 70 to 75 Nitrogen 0 to 3,
10 to 40, frequently 20 to 35. frequently 0 to 1 and for air,
frequently 30 to 35 Carbon monoxide 3 to 15, 3 to 15, frequently 3
to 10, frequently 5 to 10 and for air, frequently 3 to 6 Carbon
dioxide 10 to 25, 10 to 25, frequently 12 to 20, frequently 15 to
20 and for air, 12 to 15
[0047] As shown in FIG. 1, the reformate exits reformer 104 via
line 106. Line 106 contains splitter 118 which directs a portion of
the reformer effluent to a membrane separator, to be discussed
later, and which directs another portion to water gas shift reactor
108. In the broad aspects of this invention, a splitter is not
required in that the entire reformate stream may be directed to the
membrane separator. Where a splitter is used, generally from about
10 to 90, often from about 10 to 50, volume percent of the
reformate is directed to the membrane separator. The relative
portion of the split may vary, as stated above, to change the
relative portions of the first hydrogen product and the second
hydrogen product.
[0048] A water gas shift reactor is optional with respect to the
broad aspects of the invention. The advantage of a water gas shift
is that carbon monoxide and water are reacted to not only reduce
the concentration of carbon monoxide in the reformate but also to
generate more hydrogen. In the shift reactor 108 carbon monoxide is
exothermically reacted in the presence of a shift catalyst in the
presence of an excess amount of water to produce additional amounts
of carbon dioxide and hydrogen. The shift reaction is an
equilibrium reaction. The reformate exiting a shift reactor thus
has a reduced carbon monoxide content.
[0049] Although any number of water gas shift reaction zones may be
employed to reduce the carbon monoxide level in the hydrogen
product, the preferred processes of this invention using pressure
swing adsorption for hydrogen purification use only a high
temperature shift at high temperature shift conditions comprising
temperatures between about 320.degree. C. and about 450.degree. C.
As the hydrogen-containing stream is purified by pressure swing
adsorption, the use of more stages of water gas shift or selective
oxidation to further reduce the amount of carbon monoxide unduly
increases the expense and complexity of the hydrogen generator.
[0050] In the broader aspects of the invention, other carbon
monoxide reducing unit operations may be used such as low
temperature shift and selective oxidation to preferentially oxidize
carbon monoxide to carbon dioxide without undue combustion of
hydrogen.
[0051] The effluent from water gas shift reactor 108 is passed via
line 110 to pressure swing sorption system 112. The effluent from
water gas shift reactor 108 will also contain water and will
typically be at a temperature higher than that most advantageous
for pressure swing adsorption. Accordingly, the stream is cooled to
a temperature below about 100.degree. C., preferably to a
temperature in the range of about 300 to 80.degree. C., and most
preferably to about 35.degree. to 65.degree. C. Under these
conditions, water will be condensed and can be removed from the
stream.
[0052] The reformate is provided at an elevated pressure suitable
for pressure swing adsorption operation without additional
compression. If desired, additional compression may be
effected.
[0053] Desirably the pressure swing adsorption provides a hydrogen
product stream (the second hydrogen product) containing at least
about 90, preferably at least about 98, preferably at least about
99, volume percent. The content of impurities in the second
hydrogen product will depend upon the intended use of the product.
For use as a feed to a fuel cell, it typically will contain less
than about 20 ppmv carbon monoxide. For annealing and float glass
use, the second hydrogen product may contain nitrogen and some
minor amounts, preferably less than about 1, more preferably less
than about 0.5, volume percent carbon monoxide. For electronics
use, the second hydrogen product should have a purity of at least
about 99.999 percent including a nitrogen content of less than
about 10, preferably less than about 1, ppmv. Usually the pressure
swing adsorption recovers at least about 65, preferably at least
about 80, percent of the hydrogen contained in the stream fed to
the pressure swing adsorption.
[0054] Any suitable adsorbent or combination of adsorbents may be
used for the pressure swing adsorption. The particular adsorbents
and combinations of adsorbents used will, in part, depend upon the
components of the feed to the pressure swing adsorber, the sought
compositions in the purified hydrogen product and the geometry and
type of pressure swing adsorber used. Adsorbents include molecular
sieves including zeolites, metal oxide or metal salt, and activated
carbon. Particularly advantageous sorbents include a combination of
sorbents with the first portion of the bed being composed of
activated carbon which is particularly effective for water and
carbon dioxide removal followed by one or more molecular sieves
such as NaY, 5 A, lithium or barium exchanged X, silicalite and
ZSM-5.
[0055] The pressure swing adsorber may be of any suitable design
including rotary and multiple bed. The purging of the bed may be by
vacuum, but most conveniently for simplicity, the purge is above
ambient atmospheric pressure. A preferred pressure swing adsorption
system for low maintenance operation uses at least four fixed beds.
By sequencing the beds through adsorption and regeneration steps, a
continuous flow of purified hydrogen stream can be achieved without
undue loss of hydrogen. With at least four beds, one bed at a given
time will be adsorbing, another will be providing purge, another
will be undergoing purging and another will be undergoing
repressurization.
[0056] The operation of the pressure swing adsorber will also be
influenced by the cycle time and the ratio of the pressures for the
swing. The purge usually occurs within about 100, preferably within
about 50, say, 10 to 50, kPa above ambient atmospheric pressure.
The cycle times are selected to provide the hydrogen product of a
desired purity. For a given pressure swing adsorber system, as the
cycle times become shorter, the purity achievable increases, but
also, less hydrogen is recovered. Thus, the cycle times and
adsorber sizing can be selected for a given unit based upon the
hydrogen specification and sought recovery.
[0057] Line 116 withdraws a purge from pressure swing sorption
system 112. This purge usually contains some hydrogen and can be
combusted to provide heat within the hydrogen generator, e.g., by
preheating one or more of the feeds, generating stream, or
providing indirect heat to reformer 104, or used elsewhere. The
primary hydrogen product is withdrawn from pressure swing sorption
system via line 114 and may be used for any suitable purpose such
as a chemical reaction, providing annealing atmospheres, and the
like.
[0058] The processes and apparatus of this invention use a membrane
to provide the first hydrogen product. The retentate from the
membrane separation is thereafter subjected to the pressure swing
sorption. Since the retentate is at substantially the same pressure
as the feed to the permeator, the retentate need not be compressed.
Preferably substantially all the reformate is passed to the
permeate. Where it is desired to only subject a portion of the
reformate to the membrane separation process, a splitter can be
used as is shown in FIG. 1. In splitter 118, a portion of the
reformate (permeator feed fraction) is withdrawn via line 120.
Splitter 118 may be any suitable devise adapted to divide the
reformate stream. It may be a fixed splitter or variable splitter
such as a controllable valve.
[0059] The permeator feed fraction is introduced into permeator 122
which contains a selectively permeable membrane. The membrane may
be of any suitable type provided that it exhibits sufficient
selectivity. The variety of membrane materials range from metallic
membranes such as vanadium, tantalum, niobium, and palladium and
alloys of such elements to organic membranes such as polysulfone,
polyamide, polyimide, polycarbonate, polyketone, and the like
membranes. The purity of the first hydrogen product will depend in
part upon the membrane selected. Highly selective metal membranes
can provide a hydrogen product suitable for electronics use as well
as for fuel cell and chemical, annealing and float glass
operations. Preferably, where the permeate is used as a feed to a
fuel cell, the permeate contains less than about 20 ppmv carbon
monoxide.
[0060] The permeator feed fraction contacting the membrane is
preferably under conditions such that steam does not condense.
Depending upon the type of membrane, the temperature of the
permeator feed may need to be adjusted. Typically the metallic
membranes use elevated temperatures, e.g., from about 200.degree.
to 700.degree. C. or more, to achieve attractive permeation rates.
Advantageously, the effluent from reformer 104 may be at
temperatures suitable for use with metallic membranes. If polymeric
membranes are to be used, the temperature of the permeator feed
fraction generally must be reduced to prevent damage to the
membrane, e.g., to 175.degree. C. or less. Polymeric membranes
typically have much lower hydrogen selectivity than do metallic
membranes. Consequently, the first hydrogen product may be the
primary hydrogen product for annealing, chemical process feedstocks
and the like where greater amounts of impurities such as carbon
monoxide may be tolerable. However, fuel cells exist that have
greater resistance to carbon monoxide poisoning, and the permeate
provided by a less selective polymeric membrane may be quite
acceptable for these types of fuel cells.
[0061] The membranes in permeator 122 may be of any suitable design
including flat, spiral wound and hollow fiber. The permeator may be
designed to provide flow patterns of the permeator feed fraction
and the retentate co-current, cross-current or counter-current.
[0062] A partial pressure driving force is used to effect
permeation of hydrogen through the membrane. Accordingly, a
pressure differential is maintained across the membrane. Often the
pressure differential is at least about 200, preferably at least
about 300, kPa, and sometimes in the range of 300 to 2000 kPa. The
hydrogen partial pressure is a function of the mole fraction of
hydrogen and the pressure.
[0063] In accordance with this invention, only a portion, i.e., up
to about 50 mole percent of the hydrogen contained in the permeator
feed fraction is permeated. Thus, a substantial partial pressure of
hydrogen in on the retentate side of the membrane is maintained.
Often, the portion of the hydrogen contained in the permeator feed
fraction that is permeated is within the range of about 2 to 50,
more frequently between about 3 and 35, and sometimes between about
3 and 25, mole percent. The fact that only a small fraction of the
hydrogen permeates does not render the process of this invention
economically unattractive in that the retentate remains at high
pressure and can thus be recombined with the retained fraction of
the reformate. Moreover, the maintenance of a high hydrogen partial
pressure on the retentate side of the membrane enables reduced
membrane surface area to be used for a given amount of permeation
of hydrogen.
[0064] In the control system of the invention where the amount of
hydrogen permeated changes to control the rate of primary hydrogen
production, the rate of permeation of hydrogen can be affected by
either or both of a change in pressure differential across the
membrane and the rate reformate is provided to the membrane per
unit surface area. At higher rates of feed, the partial pressure on
the retentate side of the membrane will remain higher, thereby
increasing the rate of permeation of hydrogen, all other things
remaining the same. The rate change may also be effected by adding
or subtracting membrane surface area, e.g., putting on or taking
off membrane modules.
[0065] The retentate is passed from permeator 122 via line 124 for
recombination with the remaining fraction in line 106. The permeate
is passed via line 126 to fuel cell assembly 132. Electricity is
withdrawn from fuel cell assembly via line 134. Alternatively, the
second hydrogen product can be used as the feed to the fuel
cell.
[0066] As shown, line 126 from permeator 122 is provided with
pressure control valve 128. Also, hydrogen product demand sensor
130 is provided to determine the rate of primary hydrogen product
required. In one mode of operation, pressure control valve 128 and
splitter 118 are in communication with hydrogen product demand
sensor 130 such that the flow rate of the permeate feed fraction to
permeator 122 and the pressure differential across the membrane can
be controlled.
[0067] If desired, a compressor can be provided in line 120 and
hydrogen product demand sensor 130 can be in communication with the
compressor to change the pressure differential across the
membrane.
[0068] FIG. 2 is another hydrogen generator generally designated by
the numeral 200. In FIG. 2, the same designation number has been
given to the same components as in FIG. 1, and to the extent that
these components are the same, the discussion above is incorporated
herein.
[0069] In FIG. 2, splitter 118 is subsequent to water gas shift
reactor 108. This embodiment is particularly attractive for the use
of polymeric membranes in permeator 122. First, the water gas shift
equilibrium is temperature affected. Consequently, the temperature
of the effluent gases from the water gas shift reactor may be at a
temperature suitable for a polymeric membrane. Moreover, it is
usually desirable to reduce the temperature of the effluent from
the water gas shift reactor prior to introduction into the pressure
swing sorption system. Splitter 118 can be positioned downstream of
such cooling.
[0070] The hydrogen generator 300 of FIG. 3 is similar to that of
FIG. 2 except that low temperature water gas shift reactor 302
receives retentate from permeator 122 via line 124. As the
retentate is enriched with carbon monoxide in permeator 122, low
temperature shift reactor 302 can provide additional hydrogen in an
advantageous manner. Effluent from low temperature shift reactor
302 is passed vial line 304 for recombination with the retained
fraction in line 110 for passage to pressure swing sorption system
112.
[0071] The hydrogen generator 400 of FIG. 4 is similar to that of
FIG. 1 except that the flow to and from permeator 122 via lines 120
and 124 straddle water gas shift reactor 108, which for purposes of
this illustration is a high temperature shift reactor. The combined
reformate from water gas shift reactor 108 and retentate from
permeator 122 are combined and introduced into low temperature
shift reactor 108A. The effluent from low temperature shift reactor
108A is passed to pressure swing sorption system 112. The water gas
shift reaction is an equilibrium reaction affected by temperature.
The low temperature water gas shift will serve to further reduce
the carbon monoxide content of the reformate. As the retentate
contains an increased concentration of carbon monoxide due to the
selective permeation of some of the hydrogen, the recombined stream
has a higher concentration of carbon monoxide than the effluent
from water gas shift reactor 108 and hence the efficiency of
production of hydrogen by water gas shift reactor 108A is
enhanced.
* * * * *